1. Technical Field
The present invention relates to an oscillation circuit, an integrated circuit, and an abnormality detection method.
2. Related Art
Integrated circuits (IC) installed with an oscillation circuit are generally installed with a single oscillation circuit system and configured to operate by a clock generated in the oscillation circuit.
Such ICs installed with an oscillation circuit operate according to the clock generated in the oscillation circuit, however the IC fails to operate normally when oscillation of the oscillation circuit stops or the oscillation frequency becomes abnormal due to for example abnormalities in the IC, with the possibility of this causing fatal system problems.
Technology is therefore demanded for detecting abnormalities in the operation of oscillation circuits. For example installing a monitoring timer with a self-reset function, such as a watchdog timer (WDT) might be considered. However, in such cases if the clock of the WDT stops, the WDT is no longer able to perform self-reset.
Examples of other technology that detects abnormality in an oscillation circuit include for example technology disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-83474 and JP-A No. 8-76877. The technology disclosed in JP-A No. 6-83474 and JP-A No. 8-76877 employs an abnormality detection clock output from an abnormality detection sub oscillation circuit that performs abnormality detection on a main oscillation circuit outputting a clock for internal circuit operation.
However, in the technology of JP-A No. 6-83474 and JP-A No. 8-76877, there is the issue that abnormality detection cannot be performed in cases in which abnormality occurs in the main oscillation circuit when the sub oscillation circuit has stopped.
In an oscillation circuit provided with a main oscillation circuit and a sub oscillation circuit, the inventors of the present invention investigated providing a new sub oscillation circuit checking oscillation circuit for checking the sub oscillation circuit. However, new issues are encountered, namely that when a new oscillation circuit is provided to check the sub oscillation circuit, a small overall size cannot be achieved for the oscillation circuits, and it also becomes necessary to ascertain the operation state of the new oscillation circuit provided to check the sub oscillation circuit.